1. Field of the Invention
The present invention relates to an electronic camera having an image pickup sensor such as CMOS image pickup sensor.
2. Description of the Related Art
Conventionally, a CCD image pickup element as a solid-state image sensing device which converts the photographic light coming through a photographic lens to an electrical signal has been used for an electronic camera. However, in recent years, the number of cases in which a CMOS image pickup sensor is used has been increasing for the reason that it is inexpensive, does not require a complicated timing generation circuit, operates using only a single power source and consumes a small quantity of electric power and the like. Also, the CMOS image pickup sensor has a characteristic that it can take in only an optional area of the CMOS image pickup sensor as an image, while the CCD image pickup element does not have such characteristic. For example, Japanese Patent Laid Open Gazette 2000-196951 has proposed a technique for, prior to an emission of a flashlight, executing a previous light emission, reading an electrical signal originated from photoelectric charge stored according to the quantity of light received at the center portion of the CMOS image pickup sensor, and appropriately setting exposure and white balance of an electronic camera or a focus position of a lens based on the read electrical signal.
FIG. 10 is a view showing a state that the CMOS image pickup sensor receives a quantity of light emitted in a previous light emission, as proposed in Japanese Patent Laid Open Gazette 2000-196951.
The CMOS image pickup sensor 101 shown in FIG. 10 part (a) has plural photo detector element lines including lines from n to n+m which are respectively composed of plural arranged photo detector elements. FIG. 10 part (a) shows a block 101a that is in the center of the CMOS image pickup sensor 101, and composed of the above lines from n to n+m. Also, FIG. 10 part (b) shows timing signals n, . . . , and n+m reading the block 101a of the CMOS image pickup sensor 101 and sequentially shifted (delayed) in the direction of the time axis, and a previous light emission pulse for executing a previous light emission in the portion where these timing signals n, . . . , and n+m overlap each other.
In this CMOS image pickup sensor 101, by activating the respective plural photo detector element lines (lateral lines in FIG. 10) according to sequential timings shifted in the direction of the time axis, photographic light coming through a photographic lens is stored as photoelectric charge. Accordingly, there occurs a so-called a focus plane accumulation phenomenon in which an image pick up timing is different for each photo detector element line in the CMOS image pickup sensor 101 (the image pickup time of the upper portion is different from that of the lower portion in the CMOS image pickup sensor 101). Then, as shown in the part (b) of FIG. 10, the previous light emission in the portion where the timing signals from n to n+m are overlapping is executed, the photoelectric charge based on the photographic light composed of the external light and the reflected light of the previous light emission is evenly stored in the entire block 101a, and further, a voltage signal based on the photoelectric charge evenly stored is read, and whereby exposure and white balance of an electronic camera in a usual flashlight emission and a focus position of a lens are set.
In the above-described technique, since the photoelectric charge based on the photographic light composed of the external light and the reflected light of the previous light emission is evenly stored in the entire block 101a of the CMOS image pickup sensor 101, exposure, white balance in a usual flashlight emission and a focus position of a lens can be appropriately set. When photography based on the photographic light composed of the external light and the reflected light of the flashlight using this technique is performed, as long as the shutter time is relatively long and the time for storing photoelectric charge by the CMOS image pickup sensor is longer than the photographic time of the entire picture (1 frame), there is a timing in which all the timing signals for storing the photoelectric charge overlap each other, and therefore, a flashlight may be emitted according to such timing. However, when a high-speed shutter motion is performed and the shutter time is relatively short, there is a case that there is no timing in which all the timing signals for storing the photoelectric charge overlap each other. In this case, if a flashlight is emitted according to an arbitrary timing, the CMOS image pickup sensor stores the mixed photoelectric charge including the photoelectric charge based on the photographic light composed of the external light and the reflected light of the flashlight and the photoelectric charge based on the external light only, thereby a problem of deterioration in picture quality occurs.
Also, it is conceivable that a relatively inexpensive CMOS image pickup sensor mounted on a camera for a personal computer or a portable phone is built in an electronic camera. However, owing to a structural reason, such inexpensive CMOS image pickup sensor needs to be operated in a relatively short shutter time, and accordingly, still there is a case that there is no timing in which all the timing signals for storing the photoelectric charge overlap each other. This is a factor that prevents lowering the cost of an electronic camera.